1. Field of the Invention
The present invention relates to a carbon brush for a fuel pump, and to a method for manufacturing the same.
2. Description of the Related Art
A carbon brush for use in liquid fuel is incorporated in a fuel pump motor and slides against a commutator serving as the sliding partner in the liquid fuel. Electrical contact is unstable with a carbon brush for a fuel pump since liquid fuel exists between the brush and the commutator. For this reason, sparks are likely to be produced between the brush and the commutator, and the production of sparks is significant particularly at the back end of sliding when the brush and a commutator segment separate from each other. With conventional carbon brushes, phenol resin as the binder becomes almost entirely carbonized due to being fired at a high temperature, and the carbon particles that are a main component of the brush composition are bound with the phenol resin carbide. When sparks are produced, the phenol resin carbide undergoes fire damage due to the sparks, resulting in the dropout of carbon particles and wear of the brush.
Since the production of sparks is low when the fuel pump motor operates with a low voltage, wear is also not very significant, but when a certain voltage is exceeded, carbon dropout increases rapidly, surface abrasion occurs at the back end of sliding, and roughness increases. When this happens, sparks are more likely to be produced, surface abrasion becomes more significant, and the brush rapidly becomes more worn. As described above, when the fuel pump motor is operated at a high voltage under a high load, so-called abnormal wear occurs, that is to say, the brush becomes abnormally worn in a short time. Since the lifetime of the brush and the commutator is one of the important functions of a fuel pump, abnormal wear needs to be reliably eliminated. There is demand for fuel pump motors of recent years to be small and have a high fuel pressure, and therefore such fuel pump motors are often operated under a high load so as to rotate more quickly. For this reason, there is a rise in the risk of abnormal wear occurring, which is a point of great concern in design.
As an example of related art, Patent Document 1 (JP 2007-300748A) discloses the suppression of commutation sparks by adding 1 to 10 wt % of TiO2 to a carbon brush for a fuel pump. Also, Patent Document 2 (JP 2008-43028A) discloses the suppression of commutation sparks by adding 5 to 30 wt % of montmorillonite or fibrous potassium titanate to a carbon brush. However, through experiments performed by the inventors of the present application, it was revealed that the prevention of abnormal wear when a fuel pump motor is operated at a high voltage is insufficient with the carbon brushes of Patent Documents 1 and 2.